Intrusion detection systems are of a wide variety of types. The type with which the present invention is concerned is the type that detects human movement against a barrier.
Various systems of the foregoing general type are known. For example, taut wire detection systems of the sort described in U.S. Pat. No. 4,829,287 (Kerr et al., May 9, 1989) comprise a perimeter fence having tautly strung wires on fixed posts, which wires in turn are connected to sensors. Various suitable sensors for taut wire perimeter detection systems are known; in each case the vibration of the taut wire conveyed to a sensor element produces an output electrical signal. Suitable sensor elements described in the prior literature include materials whose resistance changes with applied force, piezo-electric crystals, and the like.
Various other intrusion detection systems are known which respond to vibrations of various kinds, even seismic vibrations, exemplary literature including U.S. Pat. No. 4,107,660 (Chleboun, Aug. 15, 1978) and U.S. Pat. No. 4,223,304 (Barowitz, Sep. 16, 1980). The Barowitz patent additionally discloses the use of a two-channel signal processing system, each channel tuned to a narrow frequency band. One channel is centred on 33 Hz and the other on 100 Hz. By contrast, Chleboun looks at frequencies of a seismic range (10 to 100 Hz) and frequencies of a pressure range (less than 1 Hz).
It is also known that so-called noisy coaxial cables may be fixed to rigid plates for use as signal generators in response to mechanical vibrations of the rigid plates. This kind of intrusion detection system is not a barrier-type system, but rather is suitable for use at such locations as railway stations in which intrusion of a human onto or in the vicinity of a railway track may result in injury to the human. As a consequence of impact on or deflection of the plate, an alarm is generated, so that oncoming trains may be halted or other remedial measures taken. Such a system is described for example in Canadian Patent No. 1,273,428 (Kerr, Aug. 28, 1990).
The GTE-Sylvania (trademark) FPS-1 intrusion detection system includes a noisy coaxial cable connected to signal-processing circuitry that responds to vibrations in an audible range, say of the order of a few hundred Hz. The Stellar E-Flex (trademark) system is similarly designed. Known coaxial cables of this type are referred to as "noisy" because of their relatively loose construction, permitting slippage of the inner and outer conductors relative to one another. Such relative movement of the inner and outer conductors causes a change in the electrical characteristics (notably the impedance) of the cable, thereby generating a detectable signal that varies in dependence upon such relative movement.
While the aforementioned intrusion detection systems all offer some degree of utility, they tend to suffer from a number of common failings.
First, many of the barrier-type systems require specially constructed barriers, generally of a rigid or semi-rigid (e.g. taut) character. Such systems cannot effectively be used with conventional physical barriers such as coiled barbed wire, chain-link fencing, or the like.
Second, the frequency ranges selected are often inappropriate for optimum detection of human intrusion.
Third, partly because special physical constituents are often required to form the barrier, such intrusion detection systems tend to be expensive to manufacture and install.
Another problem with previously known systems is the problem of false alarms, which tends to be a persistent problem difficult to remedy. The more sensitive a system is, the more likely it is to produce false alarms unless counter-measures are taken. It is known in the prior art to select for further processing only certain frequencies of interest and to eliminate at least some spurious signals by passing them through a delay circuit which rejects signals that do not persist for more than some predetermined period of time. The result is that only signals within a frequency band of interest that have an amplitude exceeding a threshold amplitude during a period of time that exceeds some threshold time interval are capable of producing an output alarm signal.